Field
The disclosed technology generally relates to chalcogenide thin films, and more particularly to ternary and quaternary chalcogenide thin films having a wide band-gap, and further relates to photovoltaic cells containing such thin films.
Description of the Related Technology
Some chalcogenide materials, such as Cu2SiS3 (CSiS), Cu2SiSe3 (CSiSe), Cu2ZnSiSe4 (CZSiSe) and Cu2ZnSiS4 (CZSiS) can be used in photovoltaic technologies, e.g., as an absorber layer in multi-junction thin film photovoltaic cells. Some multi-junction photovoltaic cells are characterized by having at least a top cell and a bottom cell, where the top cell and the bottom cell are made of materials having different optical properties, e.g., different band-gaps. For example, a multi-junction photovoltaic cell may have a top cell having an absorber layer made of a material with a wider band-gap than the absorber layer material of the bottom cell. For example, a bottom cell absorber layer with a relatively narrow band-gap, e.g., about 1 eV, (such as for example crystalline silicon or CuInSe2) can be combined with a top cell having an absorber layer with a relatively wide band-gap, e.g., in the range between about 1.7 eV and 2.2 eV and with a high optical absorption coefficient in the visible light range. As used herein, a wide band-gap refers to a band-gap greater than about 1.5 eV. Thus, the top and bottom cells may be optimized to absorb different portions of the electromagnetic spectrum, such that the resulting multi-junction photovoltaic cell absorbs a greater portion of the electromagnetic spectrum compared to a photovoltaic cell having only one type of absorber layer.
Some chalcogenide materials, such as Cu2SiS3 (CSiS), Cu2SiSe3 (CSiSe), Cu2ZnSiSe4 (CZSiSe) and Cu2ZnSiS4 (CZSiS), can be adapted to have band-gaps that may be particularly suitable as an absorber layer of such a multi-junction photovoltaic cell, e.g., as an absorber layer for a top cell. Thus, there is a need for fabrication methods for the formation of such thin film absorber layers.
Further, chalcogenide materials such as Cu2SiS3 (CSiS), Cu2SiSe3 (CSiSe), Cu2ZnSiSe4 (CZSiSe) and Cu2ZnSiS4 (CZSiS) may be economically and technologically desirable alternatives to some existing quaternary wide band-gap absorber layers currently used, such as for example CuInxGa(1-x)S2 (CIGS), CuInxGa(1-x)Se2 (CIGSe), Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe), because they don't contain relatively rare and expensive materials such as Ga and In. Furthermore, substitution of Sn by Si may offer an additional benefit of allowing absorber layers having higher band-gap values.
The ternary materials Cu2SiS3 and Cu2SiSe3 have attracted interest because of their simple structure as compared to the quaternary state-of-the-art wide band-gap absorber layer materials.
Some quaternary CIGS, CIGSe, CZTS and CZTSe thin film absorber layers for photovoltaic cells are formed using a two-stage process, wherein metallic layers, e.g., all metallic layers, are deposited first, followed by an annealing process, e.g., a single annealing process, that is performed under a selenium and/or sulfur containing atmosphere. However, using such process sequence for the formation of Cu2ZnSiSe4 or Cu2ZnSiS4 thin film layers often requires high temperatures, e.g., exceeding 600° C., due to the limited inter-diffusion of Si and Zn. Such high temperatures exceeding 600° C. are often undesirable with certain substrates that have a service temperature not exceeding 600° C. For example, some glass substrates, e.g., some soda-lime glass substrates that are widely used for thin film solar cells, have a glass transition temperature below 600° C. Thus, there is a need for methods of forming thin film absorber layers at lower temperatures, e.g., lower than 600° C.